Microwave radiation can be applied to a material in a number of ways, using single mode, multimode applicators, traveling wave applicators, slow wave applicators, fringing field applicators and through free space. Each of the aforementioned methods of coupling microwave energy into a material has its advantages and disadvantages which usually depend on the dielectric properties, size and shape, of the materials to be processed and the type of processing (batch, continuous, . . . etc.) to be performed.
Efficient microwave energy transfer is a function of many variables as processing occurs. A number of these variables are material related, e.g., the material type and density and material temperature as well as the time history of both the material temperature and the applied electric field. As the material is heated, the dielectric constant may exhibit hysteresis in temperature and electric field strength. Depending on the nature of the change of the dielectric constant, this may result in the application of a non-uniform electric field or thermal runaway, e.g., hot and cold spots within the material.
Other factors that influence coupling are related to the applicator, material geometry and size and the frequency or wavelength of the electromagnetic energy. Electromagnetic coupling depends on applicator size and geometry, material size and shape, the position of the material within the applicator, and even the relative sizes and shapes of the material and the applicator. In addition, both the applicator and material dimensions may change during heating which further complicates the efficient transfer of energy to the material.
Accordingly, a problem arises when attempting to generate a uniform microwave field across a relatively large surface for different material loads. As generally understood, if the volume of an applicator becomes too large, more than one electric field pattern can co-exist in the applicator, thereby making it multimode and introducing electric field non-uniformities. Current microwave applicators are incapable of generating a uniform microwave field across a surface that is relatively large compared to the wavelength of the radiation.
For instance, traveling wave applicators have some potential for providing uniformity. However, stray reflections, such as those that occur at the edges of a workpiece or any non-uniformity in the structure of the applicator can create standing waves leading to thermal non-uniformities. This is especially problematic in cases in which the material travels through more than one applicator and the dielectric properties of the material change depending on the processing conditions in the previous applicator.
An applicator design which shows some promise for applying uniform fields is a single mode applicator, provided that the fields can be extended over a sufficiently large region. This type of applicator can be tuned to specific electric field patterns (resonance modes) by varying the volume of the applicator.
One such approach is found in U.S. Pat. Nos. 4,507,588, 4,585,668, 4,630,566, 4,727,293, and 4,792,772 (Asmussen), all of which disclose methods and apparatuses in which a single mode resonant microwave applicator can be critically coupled by varying two separate, almost orthogonal variables, specifically the cavity length (by moving a short circuit) and the antenna position.
The Asmussen devices include a variable penetration antenna structure which acts to launch radiation into the applicator. The main advantage of the Asmussen device is that it enables complete critical coupling over a wide range of impedances (generated by the load in the applicator) and without the use of any external coupling structure. Critical coupling can thus be achieved by moving the short and the antenna appropriately.
By moving the flat part of the cavity wall (in a cylinder) in the z-direction (e.g., along the centerline of the cylinder), a wide range of electromagnetic modes can be established and maintained, even as the load varies (due to processing, e.g., temperature changing, material curing, etc.). However, one series of modes that cannot be routinely excited are length independent modes, TM.sub.xy0 and TE.sub.xy0. The resonant frequencies of these modes are only dependent on the diameter of the loaded structure. As a result, if the load changes during processing (e.g., the dielectric properties change, due to increased temperature, curing, phase change in the material and so forth), the resonant frequency in the cavity changes from an initial, fixed processing frequency, usually 2450 MHz or 915 MHz (which are the ISM bands allowed by the Federal Communication Commission (FCC)). The Asmussen devices are thus not capable of maintaining certain modes in a controlled manner, namely the length independent modes (TM.sub.xy0 and Te.sub.xy0), because these modes are dependent on the diameter of the applicator.
U.S. Pat. No. 5,471,037 (Goethal) discloses a single mode cylindrical applicator that operates in the TM.sub.02n resonant mode. The microwave applicator is designed to process monomers in order to produce prepolymers. The size of the microwave applicator is selected according to the particular monomers being processed (e.g., fixed dimension applicator). Therefore, there is no mechanism for altering the diameter of the applicator to account for substantially different loads or substantially different dielectric properties.
U.S. Pat. No. 3,461,261 (Lewis) relates to a TM.sub.02n applicator that processes threads and yarns with the workpieces passing along the central axis of the applicator. The dimensions of the microwave applicator are selected according to the materials being processed (e.g., fixed dimension applicator).
In general, to process wide objects in a continuous manner, such as a web or a sheet like product, as found in the paper industry, lumber industry (plywood) or electronics industry (in pre-impregnated cloth for circuit board manufacture), it is desirable to be able to (i) provide a uniform electric field over the entire product for uniform heating; (ii) vary the applicator to allow for variations in the dielectric properties of a continuously moving workpiece and, thus, vary the coupling of the radiation into the product; and (iii) control the microwave power reaching the product to control the temperature-time profile of the web.
The electric field pattern sustained by the TM.sub.0y0 series of modes, where y=1, 2 or greater, is oriented along the z-axis of the applicator and is of constant intensity along the entire length of the applicator for an empty cavity. This is an ideal mode for the processing of a web-like material. Referring to FIG. 1 (a mode chart), it can be seen that the TM.sub.010 mode is independent of the cavity length. Therefore, a low loss, infinitely long applicator is capable of sustaining the same electric field intensity throughout the length.
There is currently no known method to manipulate the dimensions of a microwave applicator, particularly the cross-sectional diameter, to maintain the resonance and achieve uniform heating of the material (load), using length independent modes. It should also be noted that all electromagnetic modes are dependent on a cross-sectional diameter of the microwave applicator (if the applicator is cylindrical or spheroid), and many have an additional dependence on the length of the applicator.
Accordingly, an object of the present invention is to provide a microwave applicator capable of providing an improved uniform electric and magnetic field over a wide area through the use of controlled feedback of critical material and process parameters.
It is a further object of the invention to provide a microwave applicator whereby field interactions with the material being processed are controlled through tuning of the output frequency of the microwave source.
Another object of the invention is to provide a microwave applicator capable of controlling microwave field uniformity and resonant mode tuning during material processing through the use of a controlled feedback of critical material and process parameters.
It is also an object of the present invention to provide a microwave applicator capable of applying a uniform electric and magnetic field across a sheet of material being transported therethrough in a continuous manner.
Another object of the invention is to provide a microwave applicator that provides improved uniformity of electric and magnetic fields along two dimensional space by launching the radiation into the device via more than one input.
A further object of the invention is to provide a microwave applicator capable of controlling and maintaining length independent modes (TM.sub.XY0 and TE.sub.XY0 modes)